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Is it safe to mix different types of fuels in a jet engine?
Hypothetically, if:

one got stuck somewhere at a remote airport
the jet aircraft or airliner they are flying had a 1/3 tank full of the proper jet A1 fuel
there was a tanker at the airport with 1/3 gasoline or petrol and another with diesel (the one used in trucks and passenger cars)

Would filling up the tanks and mixing all three fuels be safe for the operation of the jet engine, given that jet engines can burn any fuel?
Would the pilots expect to cover as much distance if they needed all the fuel to reach their closest destination safely to refuel?

Gas (mo-gas or AVGAS) in a jet engine would be very destructive. The best thing to do in that situation is have the correct fuel flown in or trucked in. Better to sit and wait than experience a flame-out or rapid disassembly mid-air.

No you can use gasoline in some engines in special circumstances.  The PT-6 allows it.  It's mostly bad for pumps like barrel pumps that depend on the fuel for lubrication.  Jets fuels called Wide Cut are actually kerosene avgas blends.  You can also run a PT-6 on "arctic diesel", which is an extra clean diesel used in the far north, sort of a near-kerosene.

As a back of the napkin calculation, let's say that airliner of yours has a maximum endurance of 8 hours. (Small airliners almost certainly have less; large ones definitely have more.) 1/3 of that is about 2½ hours. Leave a one hour reserve at landing, that's 1½ hours of usable fuel. 1½ hours at 500 kt gets you an operating range of 700-750 nm, or around 1300 km. If you're willing to dip into the reserve (which you shouldn't be, but probably isn't half as bad as a random mix of different fuels), you get more like 2000 km before running out of fuel. Now where's that airport located, again?

While turbine engine can be made to burn almost any fuel, a specific engine might not run with fuel it is not tuned for for several reasons:

The high pressure fuel pumps only work with stuff that is sufficiently lubricating. They don't use grease, because it would be too easily washed out at the operating pressure, and the pressure is needed because the fuel is injected directly into the combustors at the highest pressure present in the engine. This rules out too much gasoline—note that spark-ignition engines, even when they have direct injection, inject during suction, so at low pressure.
With little enough gasoline, the pumps wouldn't mind, and I heard some militaries had backup plan for diesel engines—which have the same issue—to mix gasoline with some oil as an emergency fuel. However to know how much gasoline is safe somebody would have to test it and nobody is going to sacrifice a very expensive piece of equipment that is a turbine engine for a test with so little value as this since there is no shortage of proper jet fuel.
Each fuel has different heat of combustion, so it needs to be metered differently to keep the engine within the rather narrow range of operating pressures and temperatures. If the heat of combustion is too low, the pressure won't be enough to keep the turbine spinning and the engine will flame out, and if it is too high, the pressure will exceed the capability of the compressor which will stall and the engine will either flame out or go through a cycle of stalls that will thoroughly destroy it.
The adjustment might be as “simple” as updating the firmware in the FADEC, but it is still a lot of work to do all the necessary measurements and then do all the tests for the modification and since there is no practical use, nobody did.
The length of the combustor also has to match the speed of combustion, so if that is different enough, you need a differently built engine altogether.

So the engine will probably tolerate some different fuel mixed in the tanks, but nobody can tell you how much exactly, because it wasn't tested.


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Is there currently any heat shielding and paint for speeds of up to Mach 6?
The Blackbird SR71 could travel faster than Mach 3 but its speed was limited to prevent its skin from melting. Its newer variant is said to be named the SR72 and it and other, civilian hypersonic aircraft in development are expected to travel at speeds of up Mach 6.
Is there currently any heat shielding and paint for repeated use at speeds of up to Mach 6, and travelling at the cruising altitude of the Concorde?

how high? for how long?

The space shuttle regularly entered the atmosphere at Mach 25.

the speed limit of the SR-71 was set by the maximum allowable air temperature at the inlet face of the compressor section of its engines, which was 800F. flying faster would cause this limit to be exceeded, which would start softening the compressor blades.

Titanium is used on leading edges and other hot spots ( replacing Al ). I don't know the temperature /speed limit.

I'm reasonably certain the speed limit (so to speak) on the SR-71 wasn't to prevent its skin from melting.
The hottest the skin got during flight was less than 600 C. That's definitely hot--but it's a long ways short of the melting temperature of titanium (1668 C).
Early supersonic aircraft often had control problems, because the leading edge of the air foil would cause a shock wave that separated the air flow. The control surfaces at the trailing edge little enough air flowing smoothly that they lost authority.
In the X-15, they combated this by building a vertical stabilizer that was basically a V-shape--thin at the front, but much wider at the rear:

This would induce a lot of drag at low speed, but for the x-15, low speed wasn't really a major concern.
The SR-71 took a different approach, using a pair of vertical stabilizers.

The leading edge of each stabilizer produces a V-shaped shock wave. At (approximately) the normal cruising speed, that V comes back from the leading edge of one stabilizer, and hits against somewhere close to the trailing edge of the other. Thus, we still have a solid air flow across the trailing edge (and rudder) of each, and maintain nice directional stability.
For that to work, however, the angle from the leading edge of one stabilizer to the trailing edge of the other has to (approximately) match the angle of the shock-wave formed at the leading edge. Outside of the designed speed range, that no longer happens.
On the SR-71, the vertical stabilizers are angled somewhat. This gives a (still fairly narrow) range of speeds at which the aerodynamics "work", rather than having only one specific speed. Nonetheless, the difference in separation between the top and bottom of the stabilizers isn't very large, so the range of speeds at which it works is fairly narrow. If you try to exceed that range, your stabilizers no longer work, and your control over the aircraft quickly deteriorates.
Having said all that, however, the answer is a clear "yes", if heat were to become a problem, there are materials available that can withstand substantially higher heat than titanium. One obvious example would be the Inconel X that was used as the skin for the X-15 (which flew at a bit over mach 7). Another possibility would be carbon or ceramic tiles, like those used in the Space Shuttles (or some of the other heat shielding it used, such as flexible blankets).
Those have some fairly serious shortcomings so they'd probably be avoided unless absolutely necessary. Inconel X is quite a bit heavier than titanium, and while the ceramic tiles were quite heat resistant, they were fragile mechanically, which led to a lot of maintenance work on the shuttles.

Be careful when talking about melting point :). This property is never considered during design because mechanical properties drop long before that. Also you are talking about the melting point of the pure titanium, which is never used in aerospace industry, because alloys have higher mechanical properties. And 600°C is high temperature, even for the best titanium alloys. If you want high-temperature titanium based compound, check titanium aluminide

600 C is a limit for long term exposure of titanium . It absorbs oxygen and embrittles above 650 C.At slightly higher temperatures nitrogen does the same.

@blacksmith37 - is e.g. steel better for this? I am aware it has other drawbacks.

@BambOo: I mentioned melting primarily (exclusively?) because it was mentioned in the question.But as noted, if heat exposure is problematic, there are other alloys that withstand much higher temperatures.

@TLW: "Steel" is a generic term that covers such a variety of alloys that it's nearly impossible to say much about steel in general. There are, however, high speed steels designed for use in cutting tools that routinely reach 600C, and maintain their hardness at that temperature (when properly hardened, at around 1200C). See (among many others) https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&LN=CN&NM=57 for details.

Nickel alloys ( such as various Incoloys) can be rolled to the thickness needed for skin and have the other necessary properties like strength, heat resistance , corrosion resistance. I doubt high speed steel can be rolled that thin and has difficult heat-treats.  Nickels can be age -hardened  after forming.

The SR72 is not a variant of the Lockheed SR71 but an entirely new, unmanned high speed reconnaissance and strike vehicle being proposed by Lockheed Martin. The designation is obviously intended to suggest a connection with the earlier aircraft, but the SR72 is intended to have a different, turbojet/scramjet propulsion system and a different role, not as yet clearly defined.
The SR72 is intended to make use of composite materials such as carbon fiber reinforced carbon, previously used in missile nose cones and/or ceramic materials to withstand the high temperatures generated by air friction at the intended Mach 6 speed at 80,000 ft.
The use of ablative materials or coatings on the aircraft would create a problem in that the aircraft would be enveloped in a layer of plasma at it's operating speed, which would tend to degrade the performance of on board sensors as well as radio communications.

The top speed achieved by the North American X15 was Mach 6.72 according to what I read. The heating effect on the airframe caused by air friction at this speed will depend on the altitude. Obviously the higher the altitude, the less dense the atmosphere and the less the kinetic heating effect at a given airspeed.

"according to what I read" asks for the reference. Moreover, keep in mind that the 15 flew almost in space (almost no air friction except for reentry, in which case it was comparable to a space shuttle)

Wikipedia gives a figure of Mach 6.70 at 102,000 ft, which is well within the atmosphere. The maximum altitude reached during the X15 program was 354,000 ft or 95.9 km which is beyond the Karman line according to both the NASA/USAF and FAI definitions, and therefore in space.

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Does GA flying in your own plane count toward a Airline Transport Pilot license?
The video Why the World is Running Out of Pilots discusses a pilot shortage, and notes the tortuous path to an airline transport pilot license (specifically). It claims there are 1500 flight hours needed.
I also know general aviation is in steep decline over the last 50 years, whilst the cost of GA aircraft have gone up dramatically.  I wonder if there is a connection between these two things.
If you own a small plane and fly yourself around in the "Mighty Luscombe" for fun or Youtubing, do those flight hours count toward the aforementioned 1500 hours?
Is there a floor to the minimum quality of airplane which "counts" - do you need a Cessna class, or will a light sport aircraft, experimental, parasail, or a towed glider suffice?
Does these 1500 hours differ in any way from the ~230 hours needed for a "commericial license", or can you do it all in the same Cessna?
In a general sense, would it help the pilot shortage if the government would act to promote general aviation as a hobby, growing it back to where it was?

GA aircraft aren't all that expensive. NEW ones are, but '60s-era 172s, Cherokees, and the like can be had for around the price of a new mid-range SUV or pickup.

Not a dup since I am asking about an ATPL not a CPL.  Edited to increase distinction.  @pondlife

The video isn't very clear, but most of the numbers it gives are "total time". This is cumulative, so the 1500 hours required for an ATPL includes the 250 hours required to get a CPL, which itself includes the 35-40 hours required to get a PPL plus the 15 hours required to add an instrument rating to your PPL. Also included in there somewhere will be 10 hours to add a multi-engine rating.
All loggable time counts toward the total. It doesn't matter if the aircraft is an airplane, helicopter, gyrocopter,  glider, balloon, powered parachute, etc. However, at least in the US, ultralights cannot be logged because they are legally not "aircraft".
As noted in the video, most aspiring airline pilots get flying jobs as soon as they get their CPL, most commonly as Certificated Flight Instructors. Many schools even guarantee a job as a CFI for students who graduate. Few people can afford tonrent their way to an ATPL, and it just doesn't make sense to use loans to fly when there is a shortage of pilots to take jobs where they can get paid to fly instead.

But from what the video is saying, the wage for a novice ATPL is rubbish and  you have no chance of paying back the costs/loans without sleeping in hovels and eating ramen.

Being an airline pilot isn't the way to riches @Harper, and it's hard to get into. There are enough people that can fund their own way to an ATPL and ex military pilots who can easily transition that the airlines aren't having to fund training at the moment. You can build hours in a light aircraft if you have the cashola.

Problem in the US is the "Colgan Rule" forces the Regionals to only hire 1500 hr pilots (with exceptions) so there is no path into a regional operation with 3-500 hours like there once was.  You have to slog it out flying checks or Beech 1900s to get to the 1500.  There is a severe shortage of crews at the jet Regionals with frequent canceled flights due to lack of crews.  I was acquainted with several US Regional chief pilots in my former job and they complained bitterly about how the 1500 hr requirement screwed up their recruiting.

@Harper There are plenty of flying jobs that can be used to build time needed to get to the airlines. As mentioned, many become instructors. There are also charters, cargo, ferry, crop-dusting, pipeline patrol, and many other options. The wages aren't great, but it's a lot better to get paid to fly than to be the one paying.

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Mathematical function in performance diagrams
I have taken upon myself to digitise some scanned old performance diagrams, e.g. roll distance as function of mass and OAT, TAS as function of power setting, etc. The obvious goal is to allow the computations to be performed in a computer instead of on paper (that is: almost never). I have written some tools to do measurements of the line intersections in the documents, so consider that problem solved.
However, what are the best functions to fit to the measured
coordinates? In selected cases, a straight line would be valid, but most often not. After all, the functions are typically solution to differential equations…
What functions are reasonable to adapt to the measured data?
Simple polynomial have the advantage that all measurements are used to fit one set of parameters. However, they are also well-known to produce oscillations between measured points. On the other hand, interpolating cubic polynomials have less oscillation problems but are… interpolating, so the measurement errors are not smoothed out. Furthermore, I have estimates of the measurement uncertainty of each point, so I would like to use that for fitting a global function.
Ideally, I would like a closed-formula for each type of curve, but I know that might be a tall order. My goal is to automate the computations, so the required precision is in the order of what you would produce with a pen and a ruler.
So, what do you suggest I do? My background is in computing science, but the physics behind these curves is beyond me. Any suggestions will be appreciated.
As an example I attach the "roll distance as a function of OAT, mass, and wind" for our PA-28-161. The image shows the actual document from our POH. I apologise for the Swedish labels, but this is in fact our legal document. The left panel shows OAT and pressure altitude, middle panel is take off mass, and rightmost panel shows wind.
Thanks

Though you have developed a tool... have a look at WebPlotDigitizer.

Forget about closed formulas unless you understand the physics well enough to know what the formula should look like. Personally, I would use spline curves, not polynomials.

If you have a good set of well-spaced points, a best-fit polynomial in $x^2$ or $x^3$ is usually good enough. It's true that high-degree polynomials oscillate, and are therefore unreliable, but you're safe with squares and cubes...
This site is free and quite good: MyCurveFit.

I have digitized many performance charts.  I do not have a formula.  Instead, I have very carefully tabulated the aircraft weight chart (middle) for each line.  I would get all the data for the 1050 line, then the 1025 line, 1000 line, etc.  I am OK with a little error between those values... it won't affect the end result that much.
I then enter the chart at the 1055' line and calculate for each weight till I exit the chart at the appropriate weight.  In this regard, we are creating a curve with the many points on the line.
The end result will be just as accurate, if not more accurate, than pen and paper.

I wouldn't bother with polynomials (higher than 1 :). Just tabulate the original graphs at the convenient points and use linear interpolation.
Being an aerospace engineer, I could come up with physical formulae for most things. But there are problems. First, this effectively means creating a model, which would need to be extensively validated through the original graphs. Second, exactly because of the legal status of these graphs, the pilots are supposed to use them, and they "prevail" over physical calculations.
Should there be an investigation or even a regular check, the performance calculations will be checked directly with the POH, and the argument that "my computer gave me this figure, and it has a better more accurate model" will not hold.
Furthermore, there are assumptions and simplifications in the standard graphs that for you as a computer scientist may seem to make things more complicated or even incorrect. But they make sense for pilots (who is the "target audience" for these graphs).
First, the data often refers to things the pilot can actually observe: indicated airspeed, indicated pressure altitude etc. They include known errors and idealisations in their design, some of which may not be explicitly disclosed.
Second, pilots often use "rules of thumb" to calculate source data (say, density altitude), or even some other charts from the regulator, instead of "correct" fourmulae. The POH charts may account for the resulting discrepancy (typically by being on the conservative side).
So, if you are going to use the software for the chart's original purpose, i.e. to assist the pilots, I would rather faithfully recreate the original charts in the least risky way, that is, linearly. Given that most of the graphs don't have significant curvature (many are perfectly linear), this may well be the optimal method. I would, though, visually select the most convenient points for each curve instead of using a regular grid. That is, points at the intersections with the original grid, and, of course, with higher density at higher curvature. This will minimise sampling errors.
There could be exceptions, though. For example, there is a chart for calculating cross/headwind components from direction and strength. Obviously, this is just plotted sin/cos. Or when there is an explicit reference to the International Standard Atmosphere (ISA), I would use the original well-established formulae for ISA (e.g.: the temperature gradient is exactly -6.5°/km up to 11 km; but even then you need to be careful which altitude is used here).
What works for you is that the expected accuracy is not very high (in the GA world at least). Given that the graphs are often rather small, nobody can reasonably expect to draw points better than about 1/4 of the grid cell.


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Can solo time be logged with a passenger?
Here's a very niche question. If someone were to have completed their recreational or sport pilot license and be actively pursuing their private pilot license, could flight time without an instructor but with a passenger on board (allowed because the pilot in question has their sport/recreational) be logged as solo? It's a weird question with no real cause, but it was something I was thinking about and I couldn't figure it out myself.

Not an exact dupe, (since you don't have a PPL) but the answer to this question should answer yours:  https://aviation.stackexchange.com/questions/23519/what-is-the-faa-definition-of-solo-flight.

Close enough. Looks like it would be logged as PIC.

757toga, by definition yes, of course.  However, CFI supervised "solo" cross countries are a thing so it is a very reasonable question to ask.  (i.e. if "solo" time can be logged flying with someone with MORE credentials than me, then...)

@757toga, and my only point is that "supervised solos" go directly against this. ;)   (the implication being that there are exceptions to many rules...)

@757toga, good point.  FAR 61.129 (a) (4) should probable be included in an answer.

@MichaelHall The term "supervised solo" isn't used in the regulations or by the FAA (unless I missed it, which is always possible!). The 61.129 regulation refers to solo or performing PIC duties with an instructor on board, so I think it's clear that they're two different things. There's a little more info in this answer and this legal interpretation.

@Pondlife, thanks for that.  I realize I am just muddying the waters here...  My INCORRECT assertion:  "supervised solo" time is used to fill solo requirements.  A more CORRECT summary:  PIC time under the supervision of a CFI may be substituted for solo time requirements.  (just ensure you don't log it as "training received"!)  I think we are good now.  Sorry for the confusion I introduced!

@757toga, please disregard my earlier remark, I was off track a bit... (per my comment above)

No, solo means you're alone in the aircraft. 14 CFR 61.51(d) says:

Except for a student pilot performing the duties of pilot in command
of an airship requiring more than one pilot flight crewmember, a pilot
may log as solo flight time only that flight time when the pilot is
the sole occupant of the aircraft.


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How can a plane spiral downwards uncontrolled instead of gliding straight ahead?
There have been cases (example) where a plane seems to spiral downwards. Shouldn't it glide?
I always thought you'd have a reasonable chance of landing a plane even without engines if you find some space to land on.

Reminder to answerers: Any speculation about the cause of this specific accident will be removed. We do not do accident speculation here!

An aircraft flies because it has forward speed through the air. The movement of air over the wings produces lift that holds up the plane. But there's a certain minimum speed any given aircraft has to have to keep the wings lifting. The physics of lift are complicated, but they depend on the air making its way over the wing in a relatively smooth and even way.
When the speed drops too low (or, technically, the angle of attack becomes too high), the wings can enter a stall, where the airflow separates from the wing and becomes chaotic. The wing stops producing lift. There are ways to manage a stall; the pilot needs to drop the nose, pick up speed, get the air moving over the wings properly again, and then bring it back to a more normal flight attitude.
Normally, if a plane loses the engine(s), it does indeed glide for quite a while, provided the pilots react correctly and let the nose go down enough to maintain that all-important speed. However, there can be other issues. For example, ice on the wings can ruin the aerodynamic flow and cause a stall in unexpected situations.
Under some circumstances, a stalling aircraft can get into a worse condition called a spin. That's what it sounds like you're describing -- a spin is a nose-down corkscrewing movement while the wings are stalled. It's caused by one wing stalling harder than the other, which makes the airplane enter an uncontrolled roll as it drops. Spins are harder to escape than a normal stall (and there are actually several different kinds of spin).
So, yes: If an airplane loses the engine(s), a pilot has a good chance of landing relatively safely, provided there's an open area within range to land on. A stall or spin is not an inevitability, nor even likely, but if a lot of things go wrong at the same time it could lead to a situation that the pilots may not be able to get out of.

Excellent first answer, welcome to ASE!

If a plane loses all sources of thrust and only has its kinetic energy left, why wouldn't it glide? Answer to that would be that it does climb, but not forever. Wings stall when the speed becomes too low. If the stall is too deep, it can be very difficult to recover from and a stall can develop into a spin.

How a Spin Happens:
Stall: A spin begins with a stall, this happens usually cause the plane is going too slow or the wing airfoil is offended by something like icing. If this stall happens unevenly between the wings, a spin can develop.
Yaw and Roll: When one wing stalls before the other, the aircraft begins to yaw (rotate horizontally) and roll (tilt) towards the stalled wing. The differential lift and drag between the two wings create this rotation.
Autorotation: As the aircraft continues to yaw and roll, it enters a state of autorotation, where the stalled wing continues to descend, while the other wing, which is still generating some lift, keeps the aircraft in a continuous rotation. The plane spirals downward in a corkscrew path.
Descent: In a spin, the aircraft loses altitude rapidly as it continues to rotate around its vertical axis. The descent rate is very high, making spins extremely dangerous if not recovered quickly

The question wasn't meant to speculate how this accident happened. I was wondering why an airplane doesn't keep gliding. After all, a plane has a lot of speed to begin with.

Something happened (we don't know yet), it stalled, then it was a unrecoverable stall, then it went into a flat spin

@DonQuiKong the 'lot of speed it had to begin with' is irrelevant if for some reason the plane has slowed down to stall speed (perhaps climbing too sharply, or simply neglecting the airspeed indicator for too long), or stall speed has risen (due to icing perhaps) to the plane's speed.

@DonQuiKong Stalling is precisely the behavior where a plane, for whatever reason, have lost too much speed to maintain either lift or control. The speed at which lift is lost is sometimes different than one where control is lost. So it is possible to have a high speed stall - you're too slow to maintain control but fast enough that you should have lift. But losing control puts you in a potential position to lose even more speed and thus lift if it goes on too long.

Once a stall develops into a spin, the airflow over the wings is so badly perturbed that a glide cannot be established. The pilot must immediately begin a procedure called spin recovery. This involves using the controls to tip both wings into a position where normal airflow can be re-established over them so that they can "start flying" again.
While the spin recovery is taking place, the plane is still dropping out of the sky which means a full spin recovery might consume one thousand feet or so of altitude. This means that if the plane is less than 1000 feet above the ground, there's not enough altitude to recover and the plane spins into the ground.
Every different plane design has its own spin/stall characteristics and its own published procedure for recovery, which the pilot must be able to execute flawlessly when needed, since there might not be time enough for a second try.


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Where is V15 and how do I find it?
My question: Where is V15 and how do I find it?
The question referred to in the practice test:

Refer to the figure 21, area 2 below: What is the floor of controlled airspace along V15?

The correct answer was 1200 ft AGL.

I want to thank @757toga, and @JScarry for the help on this. I made an 88 on my Drone certification that I took today (10/11/2018)


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What happened to the Boeing Y3 aircraft?
What happened to the Boeing Y3 aircraft? Did they ever produce a fully functional prototype? What is the likelihood that it will ever be produced, and when will they start production?

Boeing is currently investing a lot in the 777X which includes the technologies listed in that article, so you may consider that the Y3. Of course someday they will need an all-new aircraft for that market, but it will probably be a while before they replace the 777X.

Are you referring to the blended wing/body version?  The only other one I can think of is the SST.

The Y3 project is the third iteration within the Boeing Yellowstone project. Y1 was to be a replacement for the 737, but was put on hold after the decision to move forward with the 737-MAX. Y2 resulted in the 787. Y3 is supposed to address the 350-600+ passenger market, replacing the 777. To my knowledge, no prototype of such an aircraft has been built. Emirates was supposed to be in discussions with Boeing about this, but that was back in 2010, and since then I believe they have ordered the new 777-8X and -9X.

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Is the Airbus A380 a viable shorthaul airliner?
What is the technical, mechanical and economic feasibility of using the Airbus A380 for short haul flights of under 4 hours in its 800 plus seat configuration?  I'm talking about cost and economies of scale, frequency of repairs and service etc.

Since the A380 is extraordinarily large, only very large airports can handle it. The smaller regional airports can't, so it doesn't make any sense to serve anything other than international airports.

The hard part is to gather enough passengers and get them on and off the plane quickly. For a town with a population of tens of thousands of people, there's just not enough traffic. But A380 did fly between Paris and London for a while, which although is international, is quite a short flight.

I can’t find it now, but I know we had a question on large vs small airliners and fuel efficiency. The numbers show that smaller airliners are a lot more fuel efficient than large ones. The large ones only made up the difference by carrying enough fuel to fly long distances at cruise altitude. So it requires a very long flight for an A380 to even approach the efficiency of four flights in A320’s. It was mentioned that being one reason the A340 was discontinued because it rarely flew long enough to compete with the A330

Lufthansa is currently missing 30 rented Air Berlin planes and also the landing slots are problematic. The situation will not be become better till next year, when the get legally access to bought planes and slots from the bankcrupt Air Berlin. So in the meantime Lufthansa has started flying with the B747-400 (not the D-Version) in roughly 1:10 from Frankfurt to Berlin.

I just read that the Lufthansa has started a regular B747 service Frankfurt-Berlin, a flight of around one hour. The reason is that the significant carrier Airberlin has gone out of business, and a lot of that traffic was picked up by Lufthansa. The B747 is used as a means to increase throughput. The planes are not refueled in Berlin, the standing time is only 50 minutes. They can also not -- what is the term? -- dock at a terminal due to their size; passenger exit via stairs onto the tarmac.

I can't say if anyone has looked at a short-haul version of the A380, but we can look to the Boeing 747-400D for comparison.
In addition to removing the wingtip extensions and winglets as described in the Wikipedia article, a friend at Boeing described to me other changes needed. Boeing beefed up the wing box due to the increased landing cycles. There were also some changes to the brakes to improve cooling as the short flights didn't provide adequate time to cool the brakes passively.
It should be noted that ANA was the only customer with 19 aircraft built. They retired the last of them 3 years ago. So it would appear that there isn't much of a market for a short haul jumbo.

I once flew on a Korean 747 on a short-haul from Seoul to Jeju, both South Korea. Flight duration: 1h 10 min

@DohnJoe Yes, but which 747? The 747-400D is a very specific model.

I have to look it up, it has been a while. Currently Korean operates [Gimpo Intl. (GMP) to Jeju Intl. (CJU)] a 737 on seemingly lighter days, and a 777 (772 - BOEING 777/200) on seemingly stronger days.

It was indeed a 747-400 (aircraft type 744), in the other direction it was a 737-900 (aircraft type 739). The flights were taken in 2013.

@DohnJoe  But where did the flight go after Jeju? A lot of times there's a short hop to help fill the plane for a long leg that follows. I once caught a Pan Am 747 from LAX to SFO for a cheap price. Most of the LAX passengers and all the SFO passengers were headed to Japan which was the plane's next stop. ANA was using the 747-400Ds for nothing but short hauls.

Hard to say from a flight in 2013, however, Flight KE 1219 of Korean, which I deem comparable to the then 747 flight, operates a 777 (aircraft type 772) and seems to be only a two-stop flight.

Last year, Emirates started a flight between Dubai and Doha, only 235 miles (378 km) apart and the flight lasted 40 minutes. The flight is cancelled now, because of rather political reasons.
Compared to several other airliners (Boeing 787), A380 is less fuel efficient. The fuel efficiency of B787 is 102 mpg per seat, compared to A380 is
74 mpg per seat. The maximum seats Emirates offer on A380 is 615. At present, Emirates does not have a B787, but B787's seating capacity is around 300, when classes are implemented.
Taking the example of people traveling between UAE (Abu Dhabi and Dubai) and Doha, there is a very high demand. A single flight of A380 is more fuel efficient than two flights of B787, when a single A380 is carrying almost twice as many passengers as B787. If there is a demand, economy is better.
When a single aircraft flies instead of two, it will also decrease congestion, at the airports, and in the airspace.
However, when A380 is used on smaller routes as opposed to longer ones, it will increase its cycles faster.
Simply comparing fuel efficiency can be flawed (noted in this answer) as different phases of a flight cost different.

Emirates also operates other smaller routes besides Doha.

I'm confused. The 787 has a better mpg per seat and yet the A380 would be more economic? And the A380 does not decrease congestion compared to smaller wide body airlines, thanks to its massive wake turbulence.

"When a single aircraft flies instead of two, it will also decrease congestion, at the airports, and in the airspace." Unless the single aircraft requires special taxiways, special gates, and increased separation due to both massive wake turbulence and an unusually low approach speed.

@Sanchises I share your confusion: mpg/seat is a clear measure; the more efficient plane transports the same number of people more efficiently as far as fuel is concerned, no matter how many flights are needed.

The shortest regularly scheduled flight now of the A380 is the Kuwait - Dubai route; having flown this many times, it must work for Emirates as the flight is always full (at least on the lower deck).

@PeterA.Schneider, fuel consumption for aircraft is not so simple, because it depends on the leg distance and other factors. If somebody quotes simple numbers, they have pulled them from thin air and are quite likely comparing apples to oranges. And then fuel is not the only cost. There is landing tax and traffic control tax and crew salary and maintenance and amortization and one flight with larger aircraft comes ahead of two flights with smaller in these.

Airlines operate short hops with long range aircraft to get the flight cycles up, for verifying the results from the iron bird tests. When Singapore Airlines was flying the A340-500 direct to New York, they were also using it for the half hour hop to Jakarta. It makes zero economical sense.

Sure can. There are number of airlines on the Pacific rim areas which use 747s as puddle jumpers, making multiple short hop flights and the jet, simply because there is so much passenger travel between destinations there that an airplane like a 747 all of a sudden becomes useful for that purpose.   It’s not unreasonable to think that in the near future past your demands in that region could potential he make an A380 configured for 700-800 seat cabin arrangements a feasible option here.   I don’t know if structural reinforcement here would be needed as the above poster stated, primarily because the aircraft would take off with only a fraction of the fuel on board that without an intercontinental flight. 30,000 to 40,000 pounds of fuel would be useful on such routes, not the 380,000 to 400,000 pounds of fuel typically carried on a long transcontinental route.


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Is it possible to buy a retired Boeing 747 aircraft?
American airline companies landed their last passenger 747 jet into retirement last year citing higher costs of flying this aircraft in comparison to modern widebody twin-engine aircraft designed for the same purpose.
Is it possible to buy a passenger version of the Queen of the Skies (747-8) for use elsewhere if the costs of flying are not a problem and profitability in light of buying second hand compared to much expensive new aircraft? Will the FAA allow this if the aircraft is airworthy and for how long will Boeing keep maintaining its aircraft in terms of manufacturing spare parts and supporting it?

why do you think the FAA would care? As long as you have the paperwork to operate it and pay your taxes and other fees they couldn't care less what you own.

If you're in the upper millionaire or billionaire region of wealth and you fancy a 747, go ahead and do it. As long as you follow the proper regulations for private jets, there is no reason why you couldn't buy and fly a 747.

The oldest 747-8 is only ~8 years old. It's not that second hand ones aren't occasionally offered up for sale, but if you're restricting yourself to the -8, don't expect a lot of great deals.

Sure, the 747 is a Type Certified and airworthy aircraft, the FAA does not really care who owns it so long as they play by the rules. Boeing even sells them directly to the private market via their BBJ line.
There are even a few used ones listed for sale right now.
Some people even buy them for other purposes than flying them...

for how long will boeing keep maintaining its aircraft in light of
  manufacturing spare parts and innovating it

this is impossible to speculate on as its their decision ultimately but considering how many 747's there are you will be flying it long into the future.

yes you can get them for cheap as 250K USD in Dubai
https://www.emiratesauction.com/en/Cars/sitem.aspx?randid=124597

You can buy a stripped, worn out 747 airframe for 250K.  Do NOT expect flyable for that price.

The question specifies a 747-8.  That's a 747-200.

For sure, the 250K is not include even one of the engine as all the engines will cost million dollars.

Another striking West Coast location was a massive,
  painstakingly-disassembled and dressed 747 jetliner sprawled out over
  a small hill on the Universal Studios backlot. "You walk outside, and
  it's almost your worst nightmare of what really could happen," says
  Rick Carter. "A plane lands in your neighborhood, and just decimates
  everything."
The plane crash site was "a monumental set to put together. We had to
  buy a 747, and then we had to cut it up into pieces and have it strewn
  on the back lot, and then build houses around it. It was quite an
  undertaking."

[1]

For the neighborhood plane crash scene, the production crew bought a
  retired Boeing 747 formerly operated by All Nippon Airways as JA8147,
  with transportation costs of $2 million,[16] destroyed it into pieces,
  and built houses around them.

[2]
Steven Spielberg bought a 747 for his movie, War of the Worlds, destroyed it and filmed a scene using it.
There are some on sale here for a lowly 12,500,000$.


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How is an Airbus A380 fuselage joined and fastened together?
An Airbus A380 fuselage is manufactured in three different parts and then assembled together:

(Images courtesy of Airbus S.A.S 1994)
The fuselage functions under extreme conditions: low pressure outside and normal pressure on the inside (normal for humans, that is), and wind drag at near Mach 1 speeds.
How are the three parts joined together to complete an airtight unit? Does one fit into the other in a male-female configuration and how are they held together? Is it nuts and bolts, rivets, screws, airtight glue or a combination of them all?  Pictures would be greatly appreciated.

Not even jetliners are air-tight, nor do they need to be. They only need to maintain enough pressure that the pressurization system can make up the loss.

bleeding air from the engine to pressurize the fuselage reduces thrust and reduces fuel efficiency. Therefore the amount of air bleeded for pressurizing an airplane should be as optimal as possible, correct to the most optimal decimal point. The same applies to the air outlets and inlets for circulating air in and out of  the plane of fuselage.... Thefore everything has to be calculated no holes and exits that are not accounted for

Don't forget the 2 wings, the tail section, and landing gear, (nose, left and right). Those are also very large pieces.  Fuselages are only pressurized to 8000 feet typically, while flown at 41000, 43000, maybe even 45000 feet. Fresh air needs to be circulated, otherwise people get headaches and other symptoms from too much CO2 and not enough O2.

With approximately 19,000 rivets.
This picture is of the wing attachments, but the fuselage joints use the same attachment method.


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How much fuel is used for preheating the outer engines of a four-engine airliner that were not used for taxiing?
Many questions and observations here have highlighted the idea of using some kind of electric towing system for towing airplane to and from the runway, instead of using fuel to taxi. One of the impediments to this has been the need to preheat the engines during taxi, prior to take-off at full thrust.
Why use the inner engines of a four-engine airliner for taxi instead of the outer engines? Are the outer engines designed to not require preheating prior to full thrust?
If the outer engines do require preheating and are no different from the inner engines, how much fuel do they use to preheat the engines and would it not be cheaper to use electric taxiing then to preheat all engines?

“The major impediment to this has been the need to preheat engines during taxi prior to take off full thrust.” That statement reflects an incorrect understanding of airline operations.

Using outer engines instead of inner engines would create adverse yaw, making it harder to taxi straight and turn smoothly.  If it would be cheaper to use electric taxiing, then they would already be doing it.

Renewable energy is cheaper over time and once its paid for itself, it is free

@abelenky No it doesn't. It's still symmetric. The answer is that they use the inboard engines to reduce FOD ingestion, since they will have to run at higher thrusts and the outboards tend to hang over grass/runway parts that aren't swept as well. (Related to the lack of reversers on the A380's outboards)

How much fuel is used for preheating the outer engines of a four-engine airliner that were not used for taxiing?

It's not a lot.  The engines are only run at idle, and only for a few minutes (3-5).  Given that fuel burn at idle is only 5-10% of full power operation, and that a 4 engine planes are typically performing long haul missions (8+ hours), the warmup is less than 1/10th of 1% of the total fuel that they will burn during the entire flight.

Are the outer engines designed to not require preheating prior to full thrust?

No. Inboard and outboard engines are typically identical when it comes to the turbomachinery.  Both need the same warmup.


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Can cameras and LCD screens replace cabin windows?
Windows in aircraft create a weakness to the structure, that is why they are very small. I'm not asking about installing them in the cockpit, which would create safety pitfalls, but could the windows in the rest of the fuselage be replaced by cameras and LCD screens that provide real time images of the outside world that even passengers sitting in the middle of wide bodied aircraft can enjoy?  This could even include a sky camera for night travel.

To passengers it's really not that important if the images are realtime or even real. Like with today's CG and VR technology it's way cheaper just to simulate the view. Or better, just give passengers VR video games.

I'm not sure that this is a dupe: the other question is about replacing the cockpit windows, whereas this one is about replacing the cabin windows.

I think it's relevant given the two high profile incidents involving cabin windows recently.

@ELMOJO If you're referring to the recent uncontained engine disintegration, that by definition can risk causing all kinds of bad things to happen to the aircraft. That it happened to a window in one case (or even a few cases) is by no means a guarantee that it won't be a hydraulic system or critical control surface or control wiring the next time! The money there would probably be better spent at improving the ability of the engine cowling to contain the forces involved, than to try to mitigate the effects by making changes to portions of the airplane that just happened to be in the way.

I, for one, would be rather upset to only have a simulated view of outside. I rather enjoy the window experience.

Possible duplicate of Is it possible for a plane to use cameras and screens instead of a clear windshield?

@user3528438 a view that's out-of-sync with motion might be worse than no view at all from the a motion sickness perspective.

Better to leave the windows but offer an upcharge for VR goggles with 360 degree cameras around the plane so it looks like you're just flying through the air with the goggles on. Then you could get really amazing views of the grand canyon or the rocky mountains or whatever (on cross country continental US flights - I'm biased).

From a comfort perspective I doubt that LCD screens could replace sunlight, which is something I personally like a lot when flying.

Indeed; no reasonable backlight for an LCD screen could reproduce the quality of natural sunlight, in terms of brightness or wavelength.

It really pisses me off when folks close the windows and I am stuck in the middle or aisle seat.

Emirates Airlines have recently launched their new First Class with a similar concept.
It is only available in First Class and I doubt it has much to do with the structural implications as of now, but that could change if the idea catches on.

The virtual windows are a feature on the otherwise windowless cabin suites along the middle aisle, and project a high-definition view of the outside using real-time cameras mounted on the plane.

Mid-cabin suite.
Source: The Verge

how about that!

Going virtual and just mimicking traditional windows? They should have gone all the way and made it a full screen across the whole wall!

@JAB That might be a bit disconcerting for the passengers.

Wait till they connect the whole-floor screen to the downward pointing camera.

@immibis only if they had a really strong fan and good a/c unit installed to go with :)

@ivanivan They typically do have a really strong fan and a good A/C unit installed. :)

Note that the windows are a safety feature: You need to see outside in case of emergency, e.g. to know which side of the plane is surrounded by flames, or how deep in the water you are. This is actually the reason why the inner lighting matches the outside conditions for take-off and landing and the window shades have to be open. Even worse, the potential rescuers need to see inside to know the conditions in the plane (flames, smoke, attackers, debris etc.), so you would need to have some system that reliably works both ways.
I don't see how your hypothetical plane would get certified with even more emergency features relying on electricity and on something that's basically an entertainment system.

This is exactly what I was thinking... "what if there is an emergency and power get's cut and now greater issues to safety are created"

The opposite is true as well. Upon landing it's common for the flight crew to request you open the window shades. This is in the event of a fire/emergency inside the plane, responders can see in from the outside.

Also, in case of an emergency, it is useful for rescuers to be able to see from the outside what's happening inside the plane.

Problem #1: What gets displayed on the screen?
While I'm aware of Emirates' recent 'virtual windows,' there is a big problem with this idea in general: what should actually be displayed there depends on the location of the viewer relative to the window!
This is not an easy problem to solve - and it's effectively impossible to solve for multiple viewers looking at the same screens. You simply can't make the screen show what different viewers looking at the windows from different angles should see, even if you know the location and orientation of each viewer. The only solution to that is separate screens for each viewer - i.e. VR goggles.
I haven't been in Emirates' new suite, so I can't comment on how it feels in their case in particular, but I imagine this would be rather jarring to the viewer, especially in a simulated window like the ones Emirates is trying to use. It would feel like you're looking at a flat image on a screen, not like you're looking out a window - because that's exactly what would be happening.
Why does this problem happen?
When I move my head in space or turn my head relative to a real window, I can see different things out of that window. In the example of the windows on the side of an airplane, if my head is behind the window, I can see in front of the flight path through the window. If my head is in front of the window and I turn my head to look back at it, I can see behind the flight path. If my head is directly beside the window, I can see the view perpendicular to the flight path. The only way to do this with a screen is to track the location and orientation of my head, as VR headsets do.
While VR-style head-tracking would be sufficient for a single viewer, what two different viewers will see - even when looking at the same part of the same window - could be completely different. There is simply no solution to this problem when both viewers are observing the same screen, even if you do head-tracking, as the image displayed on the screen would need to be different for each viewer.
Problem #2: How much does all of this gear cost (in both purchase and maintenance,) how much does it weigh, and how much power does it use?
In order to properly implement this idea (assuming you want to see the real environment of the aircraft on the screens and not just something made-up or recorded,) you'll need to install both a lot of screens on the interior and a lot of cameras on the exterior, as well as computer systems to process the camera input and display it to the users in real-time (and even more complicated computer equipment if you want to actually do head-tracking and get the displayed image right.) This will also likely involve a substantial amount of software, especially in the head-tracking case.
All of this adds up to a large development cost and also a decent manufacturing cost. All of these systems will also need to be maintained, with parts being replaced periodically, whereas a piece of glass in the side of the fuselage requires relatively little maintenance, the parts cost is relatively small, and the development cost isn't huge.
Furthermore, the biggest actual problem with real windows is the weakening of the structure that you mentioned and, thus, the added weight to strengthen the fuselage. This turns out actually not to be all that much extra weight, since the fuselage is designed such that the windows are placed between the ribs that bear most of the load. So, there's a very real chance that all of the screens, cameras, and associated computers will actually end up weighing more than what the structure with real windows would have otherwise weighed, negating any fuel savings. Furthermore, while windows require little-to-no power input, the same is not true for cameras, screens, and computers. This requires even more fuel burn for the power generation.
So the screen/camera system probably isn't much, if any, more efficient than the normal windows. Overall, I would guess the TCO is more.
Problem #3: Safety
It turns out that windows are actually quite important safety devices - and not just the ones on the flight deck. In the event of an emergency - either in the air or on the ground - the people inside the aircraft need to be able to look at what's outside the fuselage. Whether it be observing structural damage to the aircraft after something like an uncontained engine failure or looking for smoke, fire, leaking fuel, or other hazards outside the fuselage prior to initiating an evacuation, the ability to see outside is safety critical.
After a crash or ditching - say, something like Asiana 214 or U.S. Air 1549, there will not be anything generating enough power to run all of those cameras, screens, and computers and there's a significant chance that they'll be damaged beyond usability anyway. Now how are you going to see where the water line is on the aircraft or the burning engine or leaking fuel on one side in order to know which doors are safe to use for evacuation?
After an uncontained engine failure that blows holes in a bunch of things that ought not have holes in them - such as on Qantas 32 - it helps to be able to send a flight crew member to the back and actually look at the damage to see what the current state of the aircraft is. This is especially true when the computers are giving the flight crew a confusing and seemingly-endless stream of error messages due to all of the damaged systems. This allows the crew to plan the most safe way to get the aircraft back on the ground given the current state of the aircraft as quickly as possible. Perhaps this would still be possible with the screens and cameras in this scenario, but there's also a good chance the failure would have damaged these systems such that they wouldn't be useable. You can still more or less guarantee that you'll be able to see out of a normal window.

I don't see the real issue with problem #1.  So what if it's like looking at a screen.  It doesn't have to exactly match the"analog" window experience

Actually there does exist a technology to mostly achieve this without using VR goggles: display with a lenticular sheet. Won't give you look from above/below parallax, but will work for left-right.

@sudorm-rfslash I don't know about you, but I can look at an LCD screen showing "outside" any time I want, and it doesn't feel like looking outside.  Flying is one of those things where you mostly have to just endure the experience until you get where you're going.  Being able to actually look outside at real outside and see the world from above the clouds is one of the little things that at least makes the flight interesting.  Replacing that with the same crappy monitor screens we all have at home is taking away one of the only special and unique things aboung being in an aircraft.

A screen image like that might be OK for someone in an aisle seat or in the center section of seats.  But for someone who always tries to get the window seat like me, I think a relatively low-resolution picture of the horizon instead of a window would be deeply unsatisfying and unsettling.  For starters, I wouldn't be able to look down at what we were passing over.  It would seem phony, and it would make me feel claustrophobic.  No thanks!

@sudorm-rfslash For people sensitive to motion nausea, having the outside view match the actual view angle is very important; the best remedy for motion sickness (including seasickness) is to fix your gaze on the horizon, so that what you see matches what you feel.

@sudorm-rfslash https://www.youtube.com/watch?v=q1mB-UvczMs a problem i see is: watch through the window 2 rows in front of you. you should see what's roughly in front of the plane, while people looking through the same window from 4 rows in front of you should see what's behind the plane. the screen cannot display both at the same time.

@Ruslan That's what Integral Imaging displays are for: https://phys.org/news/2010-08-3d-tv-future-video.html or https://www.youtube.com/watch?v=-IgUlltb8XA


text:
Were airworthiness certificates revoked for B737 Max?
Following up from this question - or more specifically Brian Drummond's comment on the accepted answer:

Or even revoked without modification, if something new is discovered about the existing components (e.g. failure modes)

When B737 Max were grounded due to suspected issues with MCAS, what happened to the Certificates of Airworthiness of individual aircraft?  Technically speaking,

something new [has been] discovered about the existing component

Per the quoted comment, this could result in the certificates being revoked.  Did it actually happen? If not, what formal measures exist that prevent somebody flying a B737 Max? (Other than common sense of course.)

Can anyone at least speak to whether this question gets the premise right? Would an airworthiness certificate be revoked if the entire model is grounded? It seems like an AC is for a specific plane, and I'm not sure if that would necessarily be taken away due to the grounding of the entire fleet.

@zymhan That's actually a good point. I'm not sure whether the type certificate can be revoked, but in this case that could possibly be the answer.

The grounding of the 737max is a Prohibition of Operation, not a revocation of Type Certificate. This is done by each jurisdiction around the world. China suspended flights two days before the FAA did.
Technically, if you could find a country that never prohibited its flight, the Max could still be used there. Repositioning flights of grounded aircraft, as to a storage location, are authorized on a case by case basis.


text:
Reclined seats for take off and landing
Normally on commercial flights, passengers are expected to bring their seats to an upright position for take off and landing. There are a few valid reasons for this.
A little while ago, on a couple flights in Upper Class on Virgin Atlantic the pre-flight announcement stated that their seats are certified for take off and landing in any state of recline.  I didn't think of it much at the time (other than, "oh, ok then"), but now thinking back I don't think I've ever encountered this on any other airline.
So my question is two-fold. First, what kind of certification was the announcement referring to? And, second, are there any other airlines that have this "feature"?
Update on response to a comment. The seat belt in that case was a standard lap belt. The seats themselves were positioned somewhat diagonally facing partially forward and were almost like bucket seats in the upright position.

Would it still not be safer to have the seats upright?  Surely the lap belt cannot work as well if the seat is not upright?

@quietflyer Absolutely, totally agree with you. I'm just curious as to what they were referring to.

@ymb1 Good point indeed.

Seat pitch also factors into it. With the ever shrinking seat pitch in economy, a reclined seat back is a hazard to the person behind. With the pitch in some business class seats, it's not an issue.

Related as noted in another comment -- https://aviation.stackexchange.com/questions/47474/why-did-my-business-class-seat-have-a-three-point-shoulder-harness -- it would be helpful or at least interesting if the question were modified to say the configuration of the seat belt at the seat the OP occupied and also of any other seats where he noticed the configuration.  Also whether the announcement only referenced (or only was able to heard by occupants of) seats in the Upper Class.

@quietflyer I updated my question with details of the seat I sat in

In normal cabin seats, the reason not to recline them for takeoff & landing is so the reclined seat won't impede egress for passengers in the row behind yours.  If the seats in question on VA wouldn't do that (ample space between rows, perhaps), then the usual driver for requiring seats to be fully upright is gone.
Presumably whatever "certification" they referred to considered the ability to get out of one's own seat in case of an evacuation.

Usually international business class seats (not just on VA, but on nearly all airlines) don't recline into the space of the seat behind them these days.

@reirab  Yeah, which then raises the question why everybody else doesn't make the same allowance as VA. Other than different seats & certifications, the answer there may be that it's preferred to say "all seats upright..." than to spell out who can & who can't. Keeping the announcements simple, and such.

For U.S. carriers, the reason is that regs don't allow it (see 121.311(e) in my answer). I suspect the reason it's not allowed in a lot of cases is what I describe at the end of my answer.

For the first part of the question, the answer will vary by jurisdiction. Generally, the carrier's flag country's regulations will be the ones that apply.
In the case of U.S. carriers, there is no certification that would allow this on a scheduled passenger airline, per 14 CFR 121.311(e):

Except as provided in paragraphs (e)(1) through (e)(3) of this section, no certificate holder may take off or land an airplane unless each passenger seat back is in the upright position. Each passenger shall comply with instructions given by a crewmember in compliance with this paragraph.

(e)(1) - (e)(3) don't exclude business-class (or any class) seating from this requirement.
Presumably, EASA and/or the UK's CAA have regulations that allow seats to be certified to take off and/or land with seat positions other than upright if such position would not impede evacuation in the event of an emergency.

For the second part of the question, Air New Zealand is another example where this is allowed in business class. With the 1-2-1 seat configuration, a reclined seat in business class is not impeding the evacuation of any other passenger. For example, in this Air New Zealand 777 pre-flight safety video, they say:

We ask that you place your tray table back, your leg rest down, and ensure your seat back is upright, unless you're in Business Premier, where you choose your own seat back position.

As for my personal opinion, if I were writing the regs, I wouldn't allow this. Lying down is simply not a good position to be in if the plane were to skid off the runway or something else happened where you needed to evacuate quickly. And most business class seats (at least all of the ones I've ridden in) are reclined by electric motors, which would not be able to reliably bring them back to the upright position in the event of an emergency, which would make it a bit more difficult for the passenger to evacuate. Most reasonably healthy people could still get out, but would be a bit slower in doing so. Less healthy people, especially the elderly and/or disabled might not be able to get out at all from the reclined position without help.


text:
How do aircraft window size compare to each other? What are some possible improvements on them?
I'd like to see a comparison about the window sizes of various commercial aircraft, maybe to give evidence of eventual progress in it over the generations.
For example, I surely know that the B-787 has the biggest ones but it seems strange to me that A350XWB has smaller ones in comparison, considering that it, too, is made with composite materials. Why this difference?
In addition, Boeing increase their window sizes between different iterations of the same model (for example, the first 747s had smaller windows than the later generations, and 777X has larger windows than the original 777), but Airbus seems to not have this approach (neo versions are equal to ceo ones).
Finally, I heard about Boeing Skyview, a very big single windows in Business variants but... is already delivered as solution? Any similar solution from Airbus?
P.s: I found only a link with similar topic but is more general and not specific to a comparison chart between models and/or difference B787 and A350 construction design related to window size

related: Why were the windows on the Concorde about the size of a hand?

Thank you both. I edit my question. I read also related question noted, but as I wrote for another one, topic is similar but my question is more general

Passenger windows or cockpit windows?

Passenger window

These questions are way too vague.  Basically you have observed that windows are sized differently and you expect someone to write a comparison paper for you, including why design engineers may have selected a particular size for each model aircraft.  You need to narrow the scope and ask a specific question

Simple flying posted a youtube video on this exact question yesterday: https://www.youtube.com/watch?v=nc8s98J0hSw
They compare different window sizes on different aircraft and describe some of the reasons for these design decisions.

Re "Passenger window"-- should be incorporated into question.

Wouldn't an improvement be not have any windows at all from a structural perspective? Have external cameras and screens instead ;) (only sort of joking here). They are a weak point in a pressurised vessel.

I specifically pointed on reasons for why B787 and A350XWB, even if both made with same advanced materials are so different in this topic... and in addition why old designed "refreshed" saw great improvements in this aspect if they are Boeing made, for example 777x and why not if made by Airbus, for example A330neo. I think that is not so vague, and in addition answers to these questions are not present in other thread on this site...

Including the 2 in the title, there are 5 questions here. That, in and of itself, makes this a difficult fit for a Q&A site. There could be 5 very good answers, one for each question - which one do you select as the "correct" or "most useful" answer? This is the SE model - one question, several answers, one selected as "best" (we hope the OP selects one...)

Older turboprops often had much larger windows (e.g. Fokker Friendship). In this niche, there seem to be a degradation...

How many airline passengers even look out the windows?  Back in the days when I flew commercial, it seemed that about 99% were glued to the in-flight "entertainment" screens.

Me!!! :-) Sincerely I hate people that "steals" me window seat and then sleep or read a magazine... Leave me the window seat and sit down on the inner ones... :-)

How aircraft windows compare:

Aircraft windows can be made bigger by improving the materials of the window or fuselage or reducing the amount of Gs the window frames get by changing the exterior shape and design of the fuselage. Another way is making the window surfaces smoother (to reduce surface friction) or changing the shape of the window itself.


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Could the Busemann's biplane concept be used in a turbofan engine?
Would incorporating the Busemann's biplane concept in the turbofan engine fan blades and engine casing make it more efficient, if fan tip speeds were allowed to move above the speed of sound by cancelling out the sonic boom and sonic shock waves?

@jjack is right. The Busemann biplane is a clever concept to have all shocks cancel each other, but unfortunately, the same canceling happens to all the lift. Therefore, a Busemann fan would not create any thrust. Also, the concept would only be workable if every other gap functions as a Busemann biplane, the rest having straight and parallel walls.

No.
The Busemann biplane is a clever concept to have all shocks cancel each other, but unfortunately, the same canceling happens to all the lift. Therefore, a Busemann fan would not create any thrust.
The key of the idea is that shocks occur only between the two wings, the outer surfaces being aligned with the external flow so no shocks are created there. This means the concept would only be workable if every other gap functions as a Busemann biplane, the rest having straight and parallel walls.
In addition, the concept only works at one specific supersonic speed which depends on the geometry of the gap between the wings. On a fan you get a continuous increase in Mach number when moving from the spoke to the fairing. At Mach = 1 the chord of the wing would have to be zero for the concept to work over the whole span.


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Can a pilot who is not a CFI give instruction?
Can a pilot (other than an ATP) who is not certificated and current as a CFI give instruction in aircraft required for the issuance of a pilot certificate, e.g. a private pilot certificate?

Anyone can give instruction, however someone like you describe may not give instruction which may be logged towards the issuance of a pilot certificate.  You still need to receive the required instruction from an authorized instructor, no matter how much another pilot may have already taught you.

As I remember it (1970's) ground instruction did not have to be given by a pilot but the FAA did have a certification as "ground instructor". A quick look at 61.211-217 appears to have not changed. So, even a non-pilot can take the "ground instruction" test and log ground school.

There are quite a few instructors in a Part 142 training center that are not CFIs.  I don't know of any training centers though that would train private pilot candidates.  These centers usually train pilots for aircraft type ratings and ATP certificates.  I mention it because it is different than privileges an ATP pilot would have.

I'm assuming that you're asking about FAA regulations. Anyone can train anyone else - how effective and safe that training is is a different question! - but only an instructor can issue endorsements, including signing off on training time, endorsing solo flight, and accepting a student pilot application (see 14 CFR 61.193 and 61.215).
This is from 14 CFR 61.51:

(h) Logging training time. (1) A person may log training time when
  that person receives training from an authorized instructor in an
  aircraft, flight simulator, or flight training device.
(2) The training time must be logged in a logbook and must:
(i) Be endorsed in a legible manner by the authorized instructor; and
(ii) Include a description of the training given, the length of the
  training lesson, and the authorized instructor's signature,
  certificate number, and certificate expiration date.

In theory, a non-CFI could train a student to perform as required by the ACS. But, a non-CFI couldn't authorize the student to fly solo, and he couldn't endorse the student's training time in his logbook. The student would end up able to fly but unable to get a certificate because he'd have no official training time and no solo time. So it wouldn't really be helping the student much: he'd have to go and do the required hours over again with a CFI anyway.

I can see how it might help a student who has the REQUIRED hours but isn't yet comfortable with all the procedures needed to pass the exams. A family member or close friend who has the experience can then give some extra training sessions that while they can't be logged as training time can help increase confidence and skill level. Combined with more official lessons to get the endorsements this may speed up the learning process at reduced cost.

@jwenting That's true, but you have to weigh those benefits of a non-CFI  against him creating, tolerating or not even noticing bad habits on the part of the student. I'm sure there are some cases where it works out, though.

I would also be concerned if the non-CFI would be opening himself up to legal consequences if there was an accident. "So, Bill, why did you do X?" "Because that's what Joe taught me to do." So Joe ends up being sued for Bill's accident

@TomMcW I doubt it. The PIC is always responsible, that's the law. And if he picked up a bad habit, the person handling his examination would fail massively if he didn't catch it.

@Pondlife true. I was going from my own experience learning to drive where I'd have benefited massively if I'd been able to drive my dad's car legally with him in the passenger seat a few hours before exam time. I was so nervous for the exam I made all kind of mistakes and failed the exam several times despite knowing everything inside and out. Some confidence building time would have helped me massively. As is, the driving school got to play that role and made quite a bit of extra money out of me :)

@jwenting where was that? Here in Oregon I had to log 100 hours with my parents (could have been any licensed driver over 21 iirc, but I did almost all with my parents). Had I taken a driver ed course, though, it would have only been 50.

@Someone-OnStrike most of Europe, and probably elsewhere. Only government licensed driving instructors are allowed to give driving instruction.

A person who holds a Ground Instructor certificate can.  Persons who hold a Basic Ground Instructor certificate may provide ground instruction, knowledge test endorsements, and ground portions of a flight review required for Sport, Recreational, or Private Pilot certificates.  Persons who hold Advanced Ground Instructor certificates can perform ground training and the required endorsements to take a knowledge tests for any airman certificate or rating, except instrument ratings.  They may also perform and sign off any airman for the ground portion of a flight review.  Instrument Ground Instructors may provide ground training, knowledge test recommendations and training and sign off for the ground portions of an instrument proficiency check.  The person who only holds a Ground Instructor certificate cannot, however, provide any flight instruction, including instruction in a ground based flight simulator or flight training device.
Airmen who hold an ATPL may give ground and flight instruction, but this is limited to air transport operations as prescribed in §61.167, unless said individual also holds as Flight Instructor certificate.

I guess the relevance of this answer to the question, depends on how literally we are to take the meaning of the word "in" -- "Can a pilot (other than an ATP) who is not certificated and current as a CFI give instruction in aircraft required for the issuance of a pilot certificate, e.g. a private pilot certificate?"


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Is it possible to use refrigeration technology to cool the skin of supersonic aircraft?
The fastest air breathing aircraft, the Blackbird SR71 flew at speeds above Mach 3.  Its engines could achieve much higher speeds but were limited by the aircraft's skin which would melt due to friction with the surrounding air should it fly faster.
Is it possible to use refrigeration technology to cool down the aircraft's skin so it can reach much higher speeds?

Wasn't the fuel being circulated in the wings to help cool them down?  If fridge technology was used, what are your ideas for cooling the liquid after being heated in the wings?

SR71 burs fuel so fast at that speed that the fuel flow cooling is sufficient. If it goes even faster or doesn't burn enough fuel, then I guess eventually it will go with some kind of consumable refrigeration e.g. evaporative cooling, e.g. liquid nitrogen.

As Niels pointed out the limiting factor for SR-71 speed was not fuselage temperature. At mach 3.3 the wings were not hot enough to even bake a pizza properly as they were only about 500 F... Titanium can take temperatures over 1000 F.

Is it possible to use refrigeration technology to cool down the aircraft's skin so it can reach much higher speeds?

Fundamentally: no.
Refrigeration technology moves heat energy around, it doesn't get rid of it. Your fridge moves heat from inside to the outside.
To get rid of it, you have two means, broadly speaking:

Heat up some mass and throw it away
Radiate it away as light/IR

Air cooling fundamentally heats up the surrounding air, and moves the now hot air away.  I believe the SR-71 used it's fuel as heat sink as well, heating the fuel before combusting it, and ultimately throwing it away.
Radiating it is inefficient at low temperatures. And it's directly dependent on surface area, which is at a premium on a plane.
So in essence you'd have nowhere to put the heat energy you removed from the skin of the aircraft...

I'd argue that circulating cold fuel under the hot skin to move the heat is a form of refrigeration, it's just not a closed-loop system. It's not the same as an AC compressor/heat pump, but the overall effect is almost the same.

I do not disagree with that. It fits with wikipedias definition of refrigeration.

The SR-71 speed was limited by the maximum allowable temperature of the air entering the first stage compressor at the front of the engine (~800F). The airframe heat load at mach 3 would far exceed the heat transfer capacity of any refrigeration system that could possibly fit inside the plane, and its weight would reduce its useful payload to zero.
A better solution might have been to coat the hottest parts of the plane with insulating tiles like those used on the space shuttle, but that technology was unavailable in the late 1950's and early '60's when the design of the SR-71 was developed.

As already pointed out in another answer, the SR-71's speed was limited by engine intake heating, its wings were fine. One can add that the main undercarriage tires in particular needed cooling by the fuel on its way to the engine, due to their close proximity to the more heat-resistant wing.
Yes it is thought practicable to refrigerate the engine intake air, though it has yet to be demonstrated in flight. Reaction Engines are currently developing the SABRE air-breathing rocket engine, which features a helium-refrigerated "pre-cooler" heat exchanger. This chills the incoming air and is expected to enable speeds up to and beyond Mach 5. The waste heat is used to drive the engine's thermodynamic cycle and pre-heat the hydrogen rocket propellant. It has attracted major funding from several international aerospace bigshots. At the time of writing (early 2020) the pre-cooler has successfully completed high-temperature testing in a purpose-built facility, and a new funding round for a prototype flight demonstrator is under way.
Note that at sufficiently high speeds, airframe heating does become a problem. Whether it might prove practicable, and to what extent, to refrigerate other key hotspots such as the fuselage nose or the wing leading edges must remain an open question.


text:
Where can I find glide ratios for commercial aircraft?
I know that light aircraft have poor glide ratios (typically 1:10) compared to commercial aircraft (typically 1:20 or above). I was wondering if there is a source where one can find the glide ratio by aircraft type.
I am particularly interested in the BA146 / RJ85 following the LaMia crash in Colombia.

The glide ratio is likely similar to the lift/drag ratio, though actual mass may have an impact on the glide ratio. A B747 wing has a L/D ratio of 17.7. I suspect that there are tables used to compute the continuous descent approach that would fit your need.

Related, regarding 787 glide ratio

20 is an extremely good ratio for an airliner. Nearly all of them have it under 20, with perhaps a few exceptions which may have it marginally higher (787 may be amongst them but it's hard to be sure).

The glide ratio is closely related to the lift/drag ratio; as lift/drag ratio is closely related to aerodynamic performance, aircraft manufacturers are very restrictive on such data and little reliable information is publicly available.
Some guesses can be made from incidences like Air Transat Flight 236. For the A330 this results somewhere around 24.
I'm not aware of any data on BA146/RJ85. As BA146/RJ85 focusses on short haul it may have a slightly lower glide ratio; 4 engines are also not favourable for a sailing plane. Still anything below 20 would be a surprise to me.

Edit: Seems I sort of mixed up the numbers. The A330 should be somewhere over 20 (quite a time since I worked on that...). Fopr the RJ85 on this forum the glide ratio is estimated as 15:1.
P.S.: Keep in mind that all these values are maximum values that can be reached under optimal conditions. Real values may be lower.

Air Transat flight 236 needed to glide 120 km and had at least 10 km of altitude at the moment the second engine flamed out. That is glide ratio of mere 12! The plane is capable of more, but the actual value is somewhere between 18 and 20. 24 is too much for a transport jet.

... and Ba146/RJ85 is a rather old design. I would expect less than 18.


text:
Can I do the EASA part 66 theory exams before starting practical training?
Is it possible to do the EASA Part 66 B1.1 exam modules first and do the practical training afterwards? Also, is it possible do the exams and training in different European countries?

I don't completely understand your question. Do you mean you want to do all the theory tests first, and do practical training later?

yes. it is possible?

I know very little about EASA regulations, but it looks like your first question is answered in EASA's Part-66 FAQ:

I have completed my EASA Part-66 modules for B1 and I have passed all the exams, but I still lack experience to get my licence. Is there
  a time limit to get the licence ? Will the certificate expire in a few
  years if I do not get the experience?
According to Commission Regulation (EU) No 1149/2011 of 21 October
  2011 (amending Regulation (EC) No 2042/2003), the basic examinations
  shall be passed and experience shall be acquired within the ten years
  preceding the application for an aircraft basic licence. The new
  regulation also states that for the purpose of time limits related to
  basic knowledge examinations, basic experience acquired before the
  Regulation applies, the origin of time shall be the date by which this
  Regulation applies, which is 01/08/2012 (which means until
  31/07/2022).

I couldn't find any clear answer for your second question, but another FAQ item might be relevant:

Where do I gain the required basic maintenance experience? Is it mandatory to gain the required maintenance experience in an EASA
  approved Part-145 organisation?
According to the AMC 66.A.30(a)(4), aircraft maintenance experience
  gained within different types of maintenance organisations (under
  Part-145, M.A. Subpart F, FAR-145, etc.) or under the supervision of
  independent certifying staff may be accepted by the competent
  authorities. This means that the aircraft maintenance experience may
  be accepted by the competent authority when such maintenance is
  performed in a maintenance organisation which does not necessarily
  hold an EASA Part-145 approval. However, it is on the competent
  authority to evaluate whether this experience is acceptable.
Consequently, please contact the competent authority where you intend
  to apply for a licence, in order to check whether the basic experience
  would be acceptable, with detailed information on the type of
  aircraft, its operation and the nature of the work.

In other words, ask the authority in your country.

"The new regulation also states that for the purpose of time limits related to basic knowledge examinations, basic experience acquired before the Regulation applies, the origin of time shall be the date by which this Regulation applies, which is 01/08/2012 (which means until 31/07/2022)"  .....so I have ten years to get   the experience


text:
Why does the U-2 use a chase car when landing?
Why does the Lockheed Martin U-2 spy plane need a car behind it during landing? Is that to help the pilot in case of excursion off the runway?

From L.A. Times: “U-2 pilots drive the chase cars and assist the landing pilots through radio calls.” Also: http://www.popularmechanics.com/military/a7818/chasing-the-u-2-spy-plane-in-a-pontiac-gto-11000804/ : “We rode along with the pilots who hop in muscle cars to tail a U-2 during the landing, radioing the spy plane's pilot to guide him or her through the delicate operation.”

There are a number of videos on YouTube from inside U-2 chase cars. They're pretty fun to watch and will give you an idea of what they do.

It is to help the pilot land the aircraft (the chase cars are also used during takeoff). The U2 is considered notoriously difficult to fly in low altitudes- a tradeoff in an aircraft optimized for very high altitude flight:

Because the U-2 was so specialized, it was amazingly difficult to fly. ... the lower the U-2 flew, the heavier its unassisted controls got, to the point where pilots literally had to brute-force the plane around during take-off and landing.
... landings were the biggest problem. The replacement for Johnson's cart-and-skid ground system was a compact setup — two centrally located landing gear in the fuselage and one detachable "pogo" gear on each wing — that turned the U-2 into little more than a 30,000-pound bicycle. Because of the high-lift wings, pilots didn't so much land the plane as fly it really close to the ground (usually about two feet), stall it, and then fall out of the sky.

The decreased visibility of the pilots, combined with the highly efficient wings and bicycle landing gear meant that unassisted landings are difficult (but certainly possible, as has been demonstrated)- thus the chase cars.
On of the chase car drivers (themselves U2 pilots) describes the drill:

... the objective is ensuring that the pilot safely landing the aircraft... calling him down to two feet... holding them off there and then the plane will just kind of settle down on the ground.

Note that the wing gears are only for takeoff - landing is done as a bicycle, falling over on the titanium wingtips at the end.

Does the U2 not have a radar altimeter?

@TomMcW: It wouldn't surprise me. a) it was designed in 1953, b) the underside of a spy plane is prime real estate. If you can squeeze an additional camera, a larger sensor, a larger lense in by getting rid of an instrument that is only used for about 10 seconds on every flight, you will, c) it's essentially a glider, weight is a prime factor, again, the radar altimeter would only be used for a couple of seconds, and the chase car solution works just fine. Remember: this is a plane that even throws away its landing gear! (The original design actually took off from a sled and landed on its …

… belly, it didn't even have the bicycle gear of the production version.)

"pilots didn't so much land the plane as fly it really close to the ground (usually about two feet), stall it, and then fall out of the sky." This is exactly how gliders land. Fly half a meter from the ground and then stall it. Of course, gliders fly somewhat slower and are somewhat lighter. But for a further glider comparison, it would be interesting to know when you are fully committed to landing the U2. I guess aborting the landing and making a go-around is not really easy with a U2. Landing as a bicycle and then falling over on the wingtip is also typical for gliders. :)

"Somewhat"???? Are you british :P ?


text:
What is this fighter aircraft parked in the South African desert?
What aircraft is parked at 27°50'24.0"S 21°35'54.4"E in the South African desert?

I'm slightly curious as to why you're asking about this thing that's barely visible on Google maps (it won't zoom in to more than the 200ft scale, at which the plane is smaller than the map pin that marks the location) and it's literally in the middle of nowhere, 40 miles from the nearest major road.

I found it while searching for the vastrap airstrip. At its peak time, South Africa had 2 means of nuclear weapon delivery: (1) Aircrafts (Canberra B12 and Hawker Siddeley Buccaneer). (2) Jericho Missiles. The image is not that of the B12 or Buccaneer. If you look closely at the right wing, it is clearly not a Mirage F1. The distance from the strip let me believe that it might not be decoy but may be a product of the Atlas Carver program or the Israel’s IAI Lavi pprogram. Since this airstrip was used to test prototypes of both programs.

PS. Click in google maps on your pull-down menu, click on the "earth" option and turn the 3D option on. Better detail.

According to the unofficial South African Air Force forum it is a fake Mirage F1, intended as a decoy. It seems it works as intended.
According to that forum, approximately 30 were built. They were constructed of metal tubing, metal plates and glass fibre. According to one post they even contained heating elements to give infrared sensors on satellites the impression that the "exhaust" was hot.

Vastrap Test Range Airfield (https://en.wikipedia.org/wiki/Vastrap)


text:
777 small wing panel moving all the time during landing
What is the small wing panel called that moves up and down all the time during landing?
See the following video:

What purpose does it serve?

@Pondlife - The posting "How does the particular piece of flap behind an engine on a B777 work?" does not clearly show which flap it is referring to like the photo here.  The same posting also says the flaperon is a high-speed aileron - why did it continue to move when the plane slowed down to landing speed?

That control surface is called a flaperon.
The flaperon functions as both a flap and an aileron to provide roll control, as with an aileron, and increased lift and drag, as with flaps. The flaperons on either wing move down together as flaps, and move up and down opposite each other as ailerons.
In the video, steady movement following that of the surrounding flaps from one position to another is an indication of operation in the role of a flap. If we could see the other wing we would see that flaperon moving in sequence. However, the short up and down movements, as opposed to the stationary flaps, is an indication of operation in the role of an aileron. If we could see the other wing we would see that flaperon moving opposite the flaperon in the video.


text:
Why does the aerodynamic center exist?
I am a glider pilot and instructor with a master’s degree in actuarial mathematics. I teach aerodynamics every winter for upcoming glider pilots.
As you surely know, the aerodynamic center is the location where the aerodynamic moment remains constant regardless of the angle of attack – the leverage of the lifting force acting through the center of pressure gets larger for decreasing angles of attack while the force acting through this location gets smaller.
Although I find it easy to understand the mechanisms behind the aerodynamic center, I struggle with understanding how it can exist? How can we be sure that there is such a point in the wing? I know that by definition it is a fixed point, but will it be exactly fixed in practice or just approximately fixed?
Is it not possible to construct an airfoil which will somehow have a discontinuity in center of pressure leverage but delivering the same lifting force in the two situations, yielding non-constant moment around the aerodynamic center?

Rolle's theorem comes to my mind...

Take a look at this question and see if anything there helps. Seems like there’s another question where PK explains this a little differently, but I can’t find it at the moment.

From my understanding the aerodynamic center only exists as a theoretical point in the linearized formulas. I think it only works because the airfoil in the normal angle of attack range has a constant lift derivative (6.28 cl per alpha) and the moment coefficient is a direct function of the lift coefficient.
In practice this point is approximately fixed until you enter the stall range (non linear, flow separations,...) or when you go supersonic (shockwaves change the pressure distribution and aerodynamic center)

Assume:

Thin airfoil
Vary with small angle of attacks
Small airspeed
Incompressible, attached flow

I hope you feel comfortable with 2 below observations:
$c_l = a_o(\alpha-\alpha_{L=0})$
This means the value of lift varies linearly with angle of attack  (*)
$c_m = m_o(\alpha-\alpha_{M=0})$
This means the value of moment at a arbitray point (say point A) on the chord line varies linearly with angle of attack  (**)
Suppose we are provied two above equations with all constants are known and the coordinate of point A. Then moment about $x_{ac}$ (unknown) is

$$ Moment \space about \space  x_{ac} = c_m.q.c^2 + c_l.q.c.(x_{ac}-x_A) $$
$$= m_o(\alpha-\alpha_{M=0}).q.c^2 + a_o(\alpha-\alpha_{L=0}).q.c.(x_{ac}-x_A)$$
$$ = (m_o.q.c^2+a_o.q.c.(x_{ac}-x_A)).\alpha + ...$$ (substitute (*) and (**)) , with q is dynamic pressure, c is chord line
Because moment about $x_{ac}$ is constant when $\alpha$ changes hence the total coefficient of $\alpha$ must be 0, then we solve the equation:
$$ m_o.q.c^2+a_o.q.c.(x_{ac}-x_A) = 0 $$
$$ m_o.c+a_o.(x_{ac}-x_A) = 0$$
$$ x_{ac} = \frac{-m_o.c}{a_o}+x_A$$
We now know that the AC exists because on the way finding it, we can find it, it is the root of the above equation which is the coordinate of the AC. It is clear that it is just approximately fixed becasue two observations at the begining are just approximate.
The AC only exist as long as two above observations are true, if they are not linear anymore because the angle of attack is greater than stall angle, you can have two angles with the same lifts but the moments about AC are different.

It exists as a mathematical abstraction for conventionally shaped aerofoils at moderate speeds. In the end it is derived from the centre of pressure. The centre of pressure is the point where the moments of the individual aerodynamic forces over the aerofoil give 0 momentum. This always exist on any aerofoil. Now, the centre of pressure is not at a constant location, it is continually shifting as a function of angle of attack, and it moves quite a lot. However the rate of movement and change in lift force is in proportion to each other, and based on that, the stable point can be found, which for conventional aerofoils at normal speeds is about 25% chord.
Note, that to get this, the proportionality between lift and CP location is a must. You can create any shaped wing cross sections (might not want to call it an aerofoil) which will not have this property and won't necessarily have an aerodynamic centre. Whether it is fit to fly is questionable though. However, they will always have a CP, as it is a simple mathematical property, and it will very likely move with angle of attack. Also note, that the AC does change location with increasing speed, for supersonic speeds it moves significantly backwards.

In potential flow theory, lift can be calculated as the linear superposition of a contribution from camber and one from angle of attack. While the camber-related part of lift is constant, the angle-of-attack related part varies linearly with this parameter. The center of pressure of the camber part is somewhere at mid-chord (details depend on the camber line; with a Joukowski airfoil the center of pressure is precisely at mid chord). The center of pressure of the angle-of-attack dependent part is at the quarter chord (the center of the area below the chordwise Birnbaum distribution of lift). The important part is the self-similarity of Birnbaum distributions for different angles of attack: The center of pressure of the angle-of-attack dependent part is constant and at 25% of chord for 2D flow and wings of large aspect ratio.

Is it not possible to construct an airfoil which will somehow have a discontinuity in center of pressure leverage[?]

Not in non viscous flow. And you want to minimize viscous effects in order to minimize drag, don't you?

All the forces generated on an airfoil are balanced near the 25% chord position, called the aerodynamic center. This location is only effected by airfoil camber, at 0 degree Angle of Attack (AoA), the Center of Pressure (CoP) moves rearward because of its camber producing lift. With positive AoA, below the stalling AoA, the CoP moves near 25% location. The forces are balanced near the 25% location on airfoil/flat plate with positive AoA, because the amount of forces is greater near the leading edge and gradually decrease to the trailing edge.

Jascker, first, welcome to [aviation.se]. Please review my acronym expansions to ensure I got them correct. When I looked up CoP in google (aviation cop) I got Changeover Point, yet I doubt that's what you mean here. Remember, many readers of [aviation.se] are simply flying enthusiasts and are neither pilots nor engineers. Please expand all acronyms on first use.

So you are saying that the Center of Pressure is at the 25% chord location at positive angle of attack?  What role then is played by the pitching moment coefficient? I'm not sure that it's really true that the Aerodynamic Center really is the point where all forces (or should we better say torques) produced by the airfoil are balanced.  If it were true, we wouldn't need to take the pitching moment coefficient into account.  On the other hand, the Center of Pressure (which moves as the angle of attack changes) is the point where all torques produced by the airfoil are balanced.

(Ctd) For a given angle-of-attack, if we specifiy the aerodynamic force vector acting at the Center of Pressure, and the location of the Center of Pressure, then we've completely described the pitch torque generated by the airfoil relative to any arbitrarily selected point.  The same is not true if we specify the aerodynamic force vector acting at the Aerodynamic Center, and the location of the Aerodynamic Center (which by definition is at 25% chord).   Because we also have to specify the pitching moment coefficient.  This answer a) appears to miss this point, and b) appears to imply that

(Ctd) This answer a) appears to miss this point, and b) appears to imply that the aerodynamic pitching torque generated by the airfoil relative to the Aerodynamic Center is always zero, at least at positive angles-of-attack.